As a synthetic resin cap (hereinafter, simply referred to as a cap) used in a container filled with a carbonated beverage, there is a cap including a top plate section and a cylindrical section extending downward from a circumferential edge of the top plate section, wherein a threaded section is formed at an inner surface of the cylindrical section (for example, see Japanese Unexamined Patent Application, First Publication No. 2002-211605).
FIG. 6 shows an example of the cap, in which the cap 31 includes a top plate section 32 and a cylindrical section 33 extending downward from a circumferential edge of the top plate section 32, and a threaded section 40 fitted onto an external thread 22 of a lip section 21 of a container 20 is formed at an inner surface of the cylindrical section 33. The container 20 is formed of polyethylene terephthalate (PET) or the like.
An inner seal projection 42 fitted into the lip section 21 of the container 20 to abut an inner surface 21a of the lip section 21 and an outer seal projection 44 abutting a portion from an opening end surface 21b of the lip section 21 to an outer surface 21c are formed at an inner surface of the top plate section 32. The inner and outer seal projections 42 and 44 abut the lip section 21 of the container 20 to seal the container 20. In particular, the outer seal projection 44 comes in contact with the container 20 with a pressing force stronger than that of the inner seal projection 42 to become a main constituent of sealing.
Since the cap 31 is used in the container 20 filled with the carbonated beverage, an inner pressure of the container 20 is increased so that the top plate section 32 is expanded and deformed upward. Since the inner seal projection 42 is also moved upward when the top plate section 32 is expanded and deformed, in consideration of this, the inner seal projection 42 is designed to abut the inner surface 21a at a low position.
FIG. 7 is an example of another cap, in which the cap 51 includes a cap main body 54 constituted of a top plate section 52 and a cylindrical section 53 extending downward from a circumferential edge of the top plate section 52, and a threaded section 60 is formed at an inner surface of the cylindrical section 33. An inner seal projection 62 configured to abut the inner surface 21a of the lip section 21 and an opening end seal projection 63 configured to abut a portion from the opening end surface 21b of the lip section 21 to the outer surface 21c are formed at an inner surface of the top plate section 52. The opening end seal projection 63 seals the lip section 21 in a state pressed by the lip section 21 of the container 20 upon capping and bent and deformed outward in a radial direction until abutting the cap main body 54 (for example, see Japanese Unexamined Patent Application, First Publication No. 2003-175948).